Manchester City captain Kolo Toure is determined to prove Manchester United boss Sir Alex Ferguson wrong.
Ferguson backed Arsenal rival Arsene Wenger's decision to sell Toure in the summer.
But the Ivory Coast defender told the News of the World: "I respect all my critics but there are always ups and downs.
"Ferguson should remember that he suffered bad times in his career as well. I had periods when I was at the very top of my game and now I have to work hard to get back there.
"The most important thing is to realise when you are having difficulty. I strongly believe that at the age of 28, my best is still to come.
"When I hear critics like Ferguson and Lee Dixon, who I learned so much from at Arsenal, it makes me play harder and work harder to prove them wrong.
"At the moment it's a great challenge to come to a lower club and help make them big.
"I stayed at Arsenal for too long and, when you do that, everything becomes a habit. Every day is the same. You become like the marble or the furniture in the house.
"And sometimes the importance of the furniture isn't always noticed. I became very relaxed and I lost my focus and my sharpness.
"Now at City there is a new expectation and pressure and it's so exciting."